Fix flattening for scrollable areas.

third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/overflow-auto-with-touch-expected.txt

 {
   "bounds": [800, 600],
   "children": [
     {
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true,
       "children": [
         {
           "position": [8, 8],
           "bounds": [300, 300],
+          "shouldFlattenTransform": false,
           "drawsContent": true,
           "children": [
             {
               "bounds": [285, 285],
+              "shouldFlattenTransform": false,
               "children": [
                 {
                   "bounds": [1000, 1000],
-                  "shouldFlattenTransform": false,
                   "drawsContent": true
                 }
               ]
             },
             {
               "bounds": [300, 300],
               "children": [
                 {
                   "position": [0, 285],
                   "bounds": [285, 15]
                 },
                 {
                   "position": [285, 0],
                   "bounds": [15, 285]
                 },
                 {
                   "position": [285, 285],
                   "bounds": [15, 15],
                   "drawsContent": true
                 }
               ]
             }
           ]
         }
       ]
     }
   ]
 }